1. Field of the Invention
The present invention relates to a rear suspension system for a bicycle. In particular, the invention relates to a variable reduction cross-linkage for a rear suspension bicycle, wherein the cross-linkage effectively extends the stroke length of the shock absorber, especially in comparison to connecting the shock absorber directly to the swing arm of the rear suspension, resulting in a smoother and plusher ride than is possible on known suspension systems.
2. Background of the Related Art
The primary structural component of a conventional two-wheel bicycle is the frame. On a conventional road bicycle, the frame is typically constructed from a set of tubular members assembled together to form the frame. For many bicycles, the frame is constructed from members commonly referred to as the top tube, down tube, seat tube, seat stays and chain stays, and those members are joined together at intersections commonly referred to as the head tube, seat post, bottom bracket and rear dropout. The top tube usually extends from the head tube rearwardly to the seat tube. The head tube, sometimes referred to as the neck, is a short tubular structural member at the upper forward portion of the bicycle which supports the front steering fork, which has the front wheel on it, and the handlebar. The down tube usually extends downwardly and rearwardly from the head tube to the bottom bracket, the bottom bracket usually comprising a cylindrical support member for the pedals and chain drive mechanism which powers the bicycle. The seat tube usually extends from the bottom bracket upwardly to where it is joined to the rear end of the top tube. The seat tube also usually functions to telescopically receive a seat post for supporting a seat or saddle for the bicycle rider to sit on. The chain stays normally extend rearwardly from the bottom bracket, and the seat stays normally extend downwardly and rearwardly from the top of the seat tube, the chain stays and seat stays being joined together with a rear dropout for supporting the rear axle of the rear wheel. The portion of the frame defined by the head tube, seat post and bottom bracket and the structural members that join those three items together can be referred to as the main front triangular portion of the frame, with the seat stays and chain stays defining a back triangular portion of the frame. The foregoing description represents the construction of a conventional bicycle frame which of course does not possess a suspension having any shock absorbing characteristics.
The increased popularity in recent years of off-road cycling, particularly on mountains and cross-country, has made a shock absorbing system in many instances a biking necessity. A bicycle with a properly designed suspension system is capable of traveling over extremely bumpy, uneven terrain and up or down very steep inclines. Suspension bicycles are less punishing, reduce fatigue and reduce the likelihood of injury to the rider, and are much more comfortable to ride. For off-road cycling in particular, a suspension system greatly increases the rider's ability to control the bicycle because the wheels remain in contact with the ground as they ride over rocks and bumps in the terrain instead of being bounced into the air as occurs on nonsuspension conventional bicycles.
Over the last several years the number of bicycles now equipped with suspension systems has dramatically increased. In fact, many bicycles are now fully suspended, meaning that the bicycle has both a front suspension and a rear suspension. Front suspensions were the first to become popular. Designed to remove the pounding to the bicycle front end, the front suspension is simpler to implement than a rear suspension and is a front suspension fork easy to retrofit onto an older model bicycle. On the other hand, a rear suspension will increase traction, assist in cornering and balance the ride. Riding a fully suspended mountain bike down a rough, rock strewn trail, or even level riding on city and country roads, provides a new degree of safety and comfort to the rider. It is in downhill riding and racing that a rear suspension is most beneficial, but even on ordinary city and country roads, a rear suspension allows the rider to look forward more safely to view traffic and road conditions without paying disproportionate attention to stones and potholes immediately below.
A number of pivoting "swing arm" suspensions have been developed for rear wheel suspensions on bicycles. In its simplest configuration, the chain stays, which on a conventional bicycle frame are rigidly mounted, are replaced by a pair of swing arms that are pivotally attached at their front ends to the main front triangular portion of the frame. The rear ends of the swing arms, which carry the rear wheel, move upward and downward in response to the rear wheel striking rocks, curbs and other obstructions. The range of movement of the swing arm usually is controlled by a shock absorber affixed between the swing arm and the main front frame.
An example of one highly successful full suspension bicycle is disclosed in the applicant's own U.S. Pat. No. 5,685,553, which relates to a rear suspension for a bicycle having a Y-shaped frame. Like many other fully suspended bicycles, the rear suspension system disclosed in the applicant's '553 patent is comprised of a main front frame, a rear swing arm (which in the '553 patent is described as a unified rear triangle), and a shock absorber which is attached directly to the frame and also directly to the rear swing arm. Because the shock absorber is attached directly to both the frame and the swing arm, the stroke length of the shock absorber is directly proportional to the movement of the swing arm.
Like all other fields of industry, the bicycle industry as a whole and bicycle racing in particular is extremely competitive, resulting in an evolution of many different designs for rear suspensions. Bicycle designers have tried to optimize the performance of the rear suspension system by, among other things, moving the pivot point of the swing arm. Such rear suspension designs can be generally categorized into one of three categories: the low pivot, the high pivot and the multiple pivot. For example, U.S. Pat. No. 5,217,241 (Girvin) discloses a low pivot rear suspension. Here, a single pivot is located relatively close to the line of the top of the run of the chain, slightly above the bottom bracket. In contrast, the high pivot rear suspension generally has the pivot position relatively high above the chain and front derailer. Multiple pivot rear suspensions commonly employ the use of one or more additional components which function nothing more than to redirect the force of the shock absorbers so that it may be placed in a novel location. Multiple pivot rear suspension systems are illustrated for example in U.S. Pat. Nos. 5,226,674 (Buell) and 5,244,224 (Busby).
In virtually all such designs, the shock absorber is connected either directly to the swing arm, or in the case of multiple pivot suspensions connected through the use of only a single link such as in Buell and Busby, so that the stroke length of the shock absorber is nonetheless directly proportional to the pivotal movement of the swing arm. While some minor alterations can be made to the shock absorber to modify its performance, such as using a stronger or lighter spring or by increasing or decreasing its fluid dampening capability, the limited stroke length of the shock absorber necessarily limits the overall performance of the suspension system.
Accordingly, an improved rear suspension system for a bicycle is desired.